The starting of three phase induction motors can be controlled by electromechanical devices that are well-known. Some currently-employed methods include starting with series-connected reactors or resistors and starting by changing the connections of the windings from an initial radial connection to a delta connection during operation.
Starting the motors using previously known devices involves some inherent inconveniences. Step-wise application of the voltage to the motor windings, for example, causes transitory electrical problems.
Power semiconductor devices are also used to progressively and continuously apply voltage to the motor windings durings starting. This allows adjustment of the RMS voltage applied to the motor windings and controls the RMS current during motor starting.
World Patent PCT WO84/04005 describes three phase alternating current induction motor starting wherein the conduction angle during start-up is limited in order to prevent non-conduction within any of the motor windings. The control of the starting torgue is is limited however since the starting voltage is greater than 40% of the powe system voltage.